Animate a ballista firing
You could use shape keys.
In Object mode select your object and in Object Data Properties give your object two shape keys. They will be named Basis and Key 1.

Select Key 1 and toggle to Edit mode. Select the middle vertices of your object. Make sure proportional editing is on (o key) and the mode is set to linear. Press g and adjust the white circle so that is covers the string (Mouse scroll wheel). Move the vertices until it looks good.

Toggle to object mode and change Key 1 Value. This value can be keyframed to create animation.

PS. If you use only a little bit geometry in your bow string then you don't even need proportional editing. You can just grab and move the vertices where you want. If you want to edit the actual bow to move while firing then proportional editing is handy.